It's a pilot test of a new system that lets your credit card company reward you with a special deal -- after it has verified that you have checked in someplace on Foursquare.

This gives you many incentives: To shop with participating merchants, to check in on Foursquare, and to use your AmEx card.

Therefore, it's a smart experiment for both companies.

For AmEx, it's an opportunity to work with a cool brand, appeal to younger people, make a splash at SXSW, and see how mobile apps and check-ins might fit into its credit card business.

For Foursquare, it's a great chance to get in with one of the biggest brands in payments -- an obvious opportunity to make money somewhere down the road.

The pilot isn't directly generating any revenue for Foursquare, cofounder and CEO Dennis Crowley tells us. But he hopes that it'll be successful, and that it could lead to future deals that could generate revenue for the startup.